Recently, minimizing drive tests (MDT) have been addressed in 3GPP RAN WG2. The ongoing work is aiming to define automatic collection of UE (user equipment) measurements, to enable easier monitoring of network performance and consequently to replace expensive drive-tests performed by operators manually. Since the functionality requires relevant input from the terminal, an involvement of the UEs is evident.
Currently, in 3GPP two MDT reporting approaches exist. One is Immediate MDT reporting. Immediate MDT reporting means that the UE needs to report MDT measurement immediately to the base station right after MDT measurement is done. In this case, the UE does not put a time stamp to the MDT measurement report, but instead the base station is required to do so based on its own time clock. The assumption behind is that MDT measurement reporting can be done successfully at any time, if capable UE wants to do it. Then, the time of receiving this MDT measurement report from UE at the base station can be used as time stamp of this MDT measurement report.
However, it is not clearly defined what “immediate” means and how to guarantee that a MDT measurement is reported “immediate enough”.
Another approach is Logged MDT reporting. In this case, an UE, which is in an idle mode, will also do some MDT measurements and report the results to the network when it gets connection to the network again. Since the base station cannot use its own time for time stamp of this kind of MDT measurement report, the UE needs to put the time stamp to every MDT measurement report. In Logged MDT reporting, a ring buffer could be provided and the ring buffer is re-used by re-writing its contents in a circular manner. In this way, old entries of measurement results are periodically deleted. However, in such a way, old entries are only deleted in case there is a new measurement.
As to immediate MDT reporting, in some cases, an UE which is configured to perform immediate MDT reporting in connected mode may not be able to report MDT measurement to the base station immediately after this MDT measurement is done, for example in case of a radio link failure (RLF), lack of uplink (UL) resource, software problems in UE, or any other reason. Consequently, there might be a case that the UE encountering such a problem may report an “outdated” data, which will be completed by “current” time information.
In this case, a relative long time period may be gone before the base station receives this MDT measurement report from the UE. If then the base station still uses its local time as the time stamp of this MDT measurement report, potential wrong result maybe got by later offline post processing to these MDT measurement reports, since the deviation on timing of MDT measurement done there will be too big.